If a plane parallel plate of good optical quality is stressed or bent by the amount a filter cell could impart, it should not introduce astigmatism. It's more likely the substrate itself is at fault.

In any event, if you have an extra pair if hands available, one could support the filter cell while the other rotates the retaining ring using a pair of jeweler's screwdriver tips. Use the largest ones which will fit in the slots. The cell supporter should strive to not pinch or otherwise asymmetrically squeeze; try to exert uniform force.

Use a piece of the "rubberized" shelf liner to grab hold of the filter. This will give you a better grip on it. Also check your kitchen; I have a tool that is plastic-coated that is used to open jars, etc. This works very well with filters!

Hi Carol,

It's not that he can't get the filter off whatever it's attached to, he wants to remove the retaining ring that holds the filter element in the filter cell. The only safe way to do this is with a spanner wrench. I had the same problem last year and wound up getting a set of these Adjustable-Arm Spanner Wrenches. Problem solved. I've since used them for a lot of other optical projects that would have otherwise been impossible. Here's another set from Edmund Optics. Please notice that the company is EDMUND and not EDMUNDS as is a common mistake on CN. Edmunds is, in fact, a company, but it has absolutely nothing to do with optics!

Sigapore is the camera captial. Go to any camera shop with a service department and I'm sure they will help you (most likely for free). They usually have a ton of spanner wrenches to fix lens (if you liked one buy it).

I used a vernier caliper. The jaws for measuring inside diameters worked great. Then a jar opener to grip the outside of the filter ring securely.

Loosened it nicely. Hope the astigmatism goes away. Otherwise its a hundred bucks down the toilet. Even at best focus the stars weren't round that DeepSkyStacker had trouble registering the subs.

EDIT: well, the filter retaining ring is nice and loose. Still got astigmatism.

since I bought it used... (I knew there was something wrong if a 2" 10nm H-A filter was going for a hundred bucks) i don't have much recourse. the seller said it worked fine on his SCT... but then SCT's have so much coma and field curvature it's likely these would have masked the astigmatism.

that, or imperfect tracking. that would also embiggen the stars and conceal the astigmatism.

oh well. should not have cheaped out. These deals always get me in the end.....

Just to make sure the filter is at fault, try partially rotating it to see if the aberration rotates with it. I find it almost strange that an element lying near the focus, where the light cone for any one image point is quite small and hence utilizing only a small area, will still distort. Either the substrate has a varying refractive index, or it is a piece of window glass.

Another test; place it in front of a bino or small refractor, and examine any suitable land or sky target. If the aperture is larger than the filter's aperture, simply mask of the outer annulus of unfiltered objective. This is a sensitive test, for it uses the full aperture. The magnification need not be high.

For me this poses a fascinating problem, for which I would strive to divine the fault.